1. Field of the Invention
The present invention relates to adjustable-length, telescoping support columns of the type having a pneumatic spring as the operating element, and more particularly to such support columns in which the telescoping members are locked together by gravity dependent locking means for easy assembly and disassembly of the column.
2. The Prior Art
Adjustable length support column units having pneumatic spring operating elements are known in the art and are commonly used in adjustable-height chairs, tables and the like. Generally, such units comprise two telescoping tubular members, an upper member attached to a surface being supported, such as a chair seat, and a lower member attached to the supporting structure for the surface, such as the pedestal of the chair. A piston-and-cylinder type pneumatic spring containing a gas or a gas-fluid mixture under high pressure is located coaxially within the tubular members, with the piston rod member being operatively connected in load-transmitting relationship to one of the tubular members and the cylinder member being similarly connected to the other tubular member. The overall length of the pneumatic spring, and thus the length of the support column, may be adjusted by manual operation of a valve mechanism internal to the spring.
Support column units of the above-described type are generally designed to permit the unit to be disassembled and reassembled in the field for the purpose of recharging or replacing the pneumatic spring after it has failed. Although the pneumatic spring of a support column unit in typical applications, such as in an adjustable-height chair, has a relatively long useful life, the other components of the unit have even longer useful lines. As such, it is more economical to recharge or replace the pneumatic spring when it fails rather than to discard the entire support column unit. Therefore, it is advantageous and desirable to provide a support column that is easy to disassemble and to reassemble.
In prior art field-reparable support column units, the piston rod member or the cylinder member of the pneumatic spring is normally secured near the bottom end of the lower tubular member by an appropriate fastener, such as a snap ring, while the other member of the pneumatic spring is normally inserted axially into the upper tubular member and is releasably retained therein by spring clips, plugs, ball detents, etc., acting between the upper tubular member and the gas spring member. Examples of prior art support column units having such releasable retention systems are described in U.S. Pat. No. 3,788,587, which issued on Jan. 29, 1974 to the assignee of the present application.
Although these prior art releasable retention systems do facilitate to some degree the assembly and disassembly of the support column unit, such systems have not been entirely satisfactory from the standpoint of cost and convenience. For example, it is relatively expensive to manufacture pneumatic springs having retention structures, such as ball detents or retaining grooves, in the cylinder member, since the cylinder member is normally constructed with relatively thin-walled tubing. In addition, where spring clips, plugs or the like are used to fasten the pneumatic spring to the upper tubular member, such small, detachable fasteners are subject to being lost upon removal and to being loosened during normal use of the support column unit. Moreover, such fasteners are generally difficult to remove and install, often requiring the use of tools. Therefore, a need clearly exists for a support column which is easy to assemble and disassemble without tools, which is of lower cost then prior art support columns and which does not require the removal of fasteners for disassembly.